, and tonight, juju chang in new york city, this is "nightline," december 12th, 2012. >>> good evening to you. i'm juju chang. tonight, a baffling portrait is emerging of that mass shooter in oregon who opened fire on a crowd shopping for some holiday cheer. this year, there's been a disturbing number of mass shootings, and investigators say while copycats may be inspired, each shooter has a distinct and chilling back story. we interviewed the 22-year-old's ex-girlfriend, who calls it mind boggling. >> reporter: chaos amidst the christmas mall rush. lines of shoppers, even children could be seen streaming out with their hands up while a gunman was still inside, announcing from behind a mask, "i am the shooter." a day later, a view of the suspect is now coming into focus. police say 22-year-old jacob tyler roberts was the masked gunman, a young man with no history of serious trouble, who allegedly stole an assault rifle from an acquaintance and set off alone on a deadly rampage. >> we do not understand the motive of this attack, except to say that there's no apparent relationship between the susp